Zirconium and Molybdenum in Individual Circumstellar Graphite Grains: New Isotopic Data on the Nucleosynthesis of Heavy Elements

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Abstract
We have analyzed 32 individual graphite grains from the Murchison meteorite for their Mo and/or Zr isotopic compositions by resonant ionization mass spectrometry. Enormous isotopic anomalies were observed in some of these grains for both elements. The data for Zr revealed the largest isotopic anomalies, with 96Zr/94Zr ratios ranging from 0.074 times to 10 times the solar value. The isotopic data on Mo show one population of graphite grains with close-to-terrestrial Mo composition in all isotopes and five grains with an s-process nucleosynthesis signature, i.e., correlated depletions in the p- and r-process isotopes. For eight grains we were able to measure both Mo and Zr isotopic compositions. Three of these eight graphite grains have s-process isotopic characteristics for both Zr and Mo, which suggests low-mass, thermally pulsed asymptotic giant branch stars as their origin. Four grains are puzzling, since they have nearly normal Mo compositions but significant anomalies in Zr, in particular, large depletions or enhancements in the 96Zr/94Zr ratio. Two of these grains have extraordinary enrichments in 96Zr, with 96Zr/94Zr ratios 10.4 ± 1.3 and 2.5 ± 0.3 times the solar system value. These enrichments are suggestive of the r-process, implying that these grains condensed from the ejecta of core-collapse supernovae, but these enrichments could also be made by the s-process if the neutron density were unusually high.